A metal element component of an automobile body or an automobile frame can be made from a metal sheet and/or a massive metal element of the automobile, such as a bolt, a spring or another massive component not made of a metal sheet. The metal element must exhibit high strength and good anti-corrosive qualities.
For increasing the metal element strength, the metal element can be heated in an oven to a temperature of 550° C. to 1200° C., and subsequently quickly cooled down to a temperature below 220° C. in an oil bath or by cooled air. In a so called “press-hardening process”, the metal element is heated to a temperature of 800° C. to 1000° C. and is formed into its end shape by pressing and then cooled in a press subsequent to the heating. The hardened metal element has a very high strength and can be used to produce relatively light metal elements for an automobile. Subsequent to the hardening, the hardened high strength metal element is made anti-corrosive by means of a surface treatment or a surface coating step.
On the way from the hardening oven to the oil bath or to the press, the heated metal element is, because the high temperature difference with the environment, subject to scaling so that the scale must be removed from the metal element before the metal element is passed to the surface treatment or surface coating step. A method for scale removing is pickling in a cleaning liquid with a strong acid, for example, with sulphuric acid. With this scale removing method, even the smallest structures, such as slits, hollow spaces, undercuts etc. are removed. Use of a strong acid has the disadvantage, however, that the treated high strength metal element becomes brittle. An alternative method for scale removing is shot blasting which does not affect the inner structure of the metal element but is not suitable to remove scale from small structures, slits, undercuts etc. Shot blasting stream also has the disadvantage that it can mechanically deform a sheet-made metal element.